Known hydrophone arrangements used in remote sensing applications, such as in sea bed arrays and underwater arrays, for example, typically make use of piezoelectric devices that require local active in instrumentation. This renders these prior art arrangements more susceptible to detection by unauthorised parties. They also have data and power cabling and pre-amp requirements that make such prior art hydrophone arrays bulky, difficult to deploy and maintain.
In an attempt to address these issues, hydrophones based on fibre optic technology have been developed. For example, it is known to wind a relatively large amount of optic fibre around a bobbin, and subsequently measure effects caused by the stress on this fibre to make pressure determinations. However, known methodologies and apparatus for implementing fibre optic technology in hydrophones result in bulky and complex equipment, such equipment often requiring multiple optical fibres and fibre couplings. Additionally, the complexity of such devices makes it difficult to manage a large number of devices in combination.
Another issue that hinders the applicability of known optical sensing equipment to hydrophone applications is commonly referred to as depth pressure compensation. For example, an acoustic signal that is to be detected by a sensor is typically a small pressure wave that will be referred to herein as “dynamic pressure”. By way of non-limiting example, a typical dynamic pressure wave has an amplitude of up to around 100 Pa and is in a frequency range of between approximately 10 Hz and 20 kHz. When submerged, the sensor is additionally subject to a hydrostatic pressure that will be referred to herein as “static pressure”. In the ocean this static pressure typically increases by approximately 100,000 Pa for every 10 m of depth. Hence, at depth, the static pressure is typically many orders of magnitude larger than the dynamic pressure. Unfortunately, it is typically the dynamic pressure that is of interest. The difference in magnitude between static and dynamic pressures can cause a hydrophone or similar sensor system to saturate and become deaf to the significantly smaller dynamic pressure-consequently the desired signal is insufficiently detected or in some cases not detected at all. La some cases, the static pressure affects optical equipment in a manner that necessitates the usage of a relatively large optical bandwidth to transmit the resultant signal along an optical fibre cable. These and other complications are compounded as a number of sensors are used in combination.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.